[PATCH] [LoopUnrollRuntime] Avoid high-cost trip count computation.

[Sanjoy Das]

Hi atrick,

Runtime unrolling of loops needs to emit an expression to compute the
loop's runtime trip-count.  Avoid runtime unrolling if this computation
will be expensive.

Depends on D8993.

http://reviews.llvm.org/D8994

Files:
  lib/Transforms/Utils/LoopUnrollRuntime.cpp
  test/Transforms/LoopUnroll/high-cost-trip-count-computation.ll
  test/Transforms/LoopUnroll/runtime-loop.ll
  test/Transforms/LoopUnroll/runtime-loop4.ll

Index: lib/Transforms/Utils/LoopUnrollRuntime.cpp
===================================================================
--- lib/Transforms/Utils/LoopUnrollRuntime.cpp
+++ lib/Transforms/Utils/LoopUnrollRuntime.cpp
@@ -312,6 +312,12 @@
   if (isa<SCEVCouldNotCompute>(TripCountSC))
     return false;
 
+  BasicBlock *Header = L->getHeader();
+  const DataLayout &DL = Header->getModule()->getDataLayout();
+  SCEVExpander Expander(*SE, DL, "loop-unroll");
+  if (Expander.isHighCostExpansion(TripCountSC, L))
+    return false;
+
   // We only handle cases when the unroll factor is a power of 2.
   // Count is the loop unroll factor, the number of extra copies added + 1.
   if (!isPowerOf2_32(Count))
@@ -332,18 +338,15 @@
   auto *DT = DTWP ? &DTWP->getDomTree() : nullptr;
 
   BasicBlock *PH = L->getLoopPreheader();
-  BasicBlock *Header = L->getHeader();
   BasicBlock *Latch = L->getLoopLatch();
   // It helps to splits the original preheader twice, one for the end of the
   // prolog code and one for a new loop preheader
   BasicBlock *PEnd = SplitEdge(PH, Header, DT, LI);
   BasicBlock *NewPH = SplitBlock(PEnd, PEnd->getTerminator(), DT, LI);
   BranchInst *PreHeaderBR = cast<BranchInst>(PH->getTerminator());
-  const DataLayout &DL = Header->getModule()->getDataLayout();
 
   // Compute the number of extra iterations required, which is:
   //  extra iterations = run-time trip count % (loop unroll factor + 1)
-  SCEVExpander Expander(*SE, DL, "loop-unroll");
   Value *TripCount = Expander.expandCodeFor(TripCountSC, TripCountSC->getType(),
                                             PreHeaderBR);
   Value *BECount = Expander.expandCodeFor(BECountSC, BECountSC->getType(),
Index: test/Transforms/LoopUnroll/high-cost-trip-count-computation.ll
===================================================================
--- /dev/null
+++ test/Transforms/LoopUnroll/high-cost-trip-count-computation.ll
@@ -0,0 +1,27 @@
+; RUN: opt -S -unroll-runtime -loop-unroll < %s | FileCheck %s
+
+target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64-S128"
+
+;; Check that we don't emit expensive instructions to compute trip
+;; counts when unrolling loops.
+
+define i32 @test(i64 %v12, i8* %array, i64* %loc) {
+; CHECK-LABEL: @test(
+; CHECK-NOT: udiv
+entry:
+  %step = load i64, i64* %loc, !range !0
+  br label %loop
+
+loop:                                           ; preds = %entry, %loop
+  %k.015 = phi i64 [ %v15, %loop ], [ %v12, %entry ]
+  %v14 = getelementptr inbounds i8, i8* %array, i64 %k.015
+  store i8 0, i8* %v14
+  %v15 = add nuw nsw i64 %k.015, %step
+  %v16 = icmp slt i64 %v15, 8193
+  br i1 %v16, label %loop, label %loopexit
+
+loopexit:                             ; preds = %loop
+  ret i32 0
+}
+
+!0 = !{i64 1, i64 100}
Index: test/Transforms/LoopUnroll/runtime-loop.ll
===================================================================
--- test/Transforms/LoopUnroll/runtime-loop.ll
+++ test/Transforms/LoopUnroll/runtime-loop.ll
@@ -1,5 +1,7 @@
 ; RUN: opt < %s -S -loop-unroll -unroll-runtime=true | FileCheck %s
 
+target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64-S128"
+
 ; Tests for unrolling loops with run-time trip counts
 
 ; CHECK: %xtraiter = and i32 %n
Index: test/Transforms/LoopUnroll/runtime-loop4.ll
===================================================================
--- test/Transforms/LoopUnroll/runtime-loop4.ll
+++ test/Transforms/LoopUnroll/runtime-loop4.ll
@@ -20,7 +20,8 @@
   br label %loop2.header
 
 loop2.header:
-  br label %loop2
+  %e = icmp uge i32 %iter, 1
+  br i1 %e, label %loop2, label %exit2
 
 loop2:
   %iv2 = phi i32 [ 0, %loop2.header ], [ %inc2, %loop2 ]

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